Transfer apparatus having vacuum holes and method of making such apparatus

ABSTRACT

A transfer apparatus includes a transfer member, for example, a transfer drum with a line of small closely spaced laser drilled vacuum holes to hold a receiving sheet to the drum surface. To maintain the continuity of a transfer field but be free from clogging, the holes are narrow near the surface of the drum but wider as they extend deeper in the drum. 
     Conical holes are drilled in the drum by a laser. In one embodiment, a short focal length lens focuses a laser bear near the surface of the drum which beam spreads below the surface to form a conical hole.

RELATED APPLICATIONS

This application is related to co-assigned:

U.S. patent application Ser. No. 375,105; now U.S. Pat. No. 4,949,129,issued Aug. 14, 1990, filed July 3, 1989. APPARATUS FOR TRANSFERRINGTONER IMAGES TO A RECEIVING SHEET, William Y. Fowlkes et al.

U.S. patent application Ser. No. 375,240 filed July 3, 1989, TRANSFERAPPARATUS HAVING A TRANSFER MEMBER WITH VACUUM MEANS, Marcus S. Bermelet al.

U.S. patent application Ser. No. 375,165; now U.S. Pat. No. 4,941,020,issued July 10, 1990. filed July 3, 1989, TRANSFER APPARATUS HAVINGVACUUM HOLES FOR HOLDING A RECEIVING SHEET, Richard C. Baughman et al.

TECHNICAL FIELD

This invention relates to apparatus for transferring electrostaticallyheld toner images to a receiving sheet. More specifically, thisinvention relates to such apparatus including a transfer drum havingvacuum holes or the like for holding the receiving sheet as it passesthrough transfer relation with a toner image. It also relates to amethod of making such a transfer drum.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,712,906, Bothner et al, shows an electrophotographiccolor printer which forms consecutive images in different colors thatare transferred in registry to a receiving sheet. The receiving sheet iswrapped on a transfer drum or roller and recirculated on the surface ofthe drum into transfer relation with the consecutive images to create amulticolor image on the sheets. To improve efficiency, large sheets, forexample, "ledger" size sheets are placed on the drum with the smalldimension parallel to the axis of the drum and wrapped substantiallyaround the transfer drum. Small sheets, for example, "letter" sizesheets are placed with their long dimension parallel to the axis of thedrum. Since the short dimension of letter size sheets is approximatelyhalf the long dimension of ledger size sheets, two letter size sheetsare placed on the drum in approximately the same space as the singleledger size sheet.

Prior to the Bothner invention, commercial color image transfer devicessecured the receiving sheet to the transfer drum with small grippingfingers that grip the leading edge of the sheet. Many other methods arementioned in the literature, for example, vacuum holes, electrostaticsor various combinations of vacuum holes, electrostatics and grippingfingers. The gripping fingers were preferred commercially because theymore firmly hold the sheet against slippage, which slippage woulddegrade the registration of the color images.

However, the Bothner invention is difficult to utilize with grippingfingers because the leading edge of the second letter size sheet ispositioned at approximately the middle of a ledger size sheet. For someapplications, retractable fingers may be made to work, but for manyapplications they would leave substantial image artifacts in a ledgersize sheet. Bothner therefore suggests the use of vacuum holes which arepositioned at the leading edge of each of the smaller sheets and may ormay not both be activated for the ledger size sheet.

To firmly hold fairly heavy stock the holes were made as large as 3-6 mmin diameter and placed less than one to a centimeter in a line acrossthe drum.

The vacuum holes shown in Bothner work fine in many situations. However,under some conditions, the vacuum holes show up on the final image assmall round areas of incomplete toner transfer. This is especially truein dry ambient conditions, with transparency receiving stock and withthe second transfer to duplex receiving sheets where the receiving sheethas been dried by a prior fusing step.

Even in dry conditions, the artifacts may be acceptable if they wereconfined to the leading edge of all sheets, where image information isunlikely. However, the Bothner apparatus forces at least one line ofvacuum holes, for the leading edge of the second small sheet, to themiddle of a large sheet. Further, in different sheet holdingapplications, it may be necessary to put vacuum holes at the trailingedge as well as the leading edge of at least some sheets. If a varietyof sheet sizes is to be available, many lines of trailing edge holeswill be necessary. Vacuum holes on the trailing edges of a variety ofsheets place many lines of holes in the middle of larger sheets,depending on the mixture of sizes available in the machine.

U.S. Pat. No. 4,080,053, Friday, shows a vacuum web transport for a copysheet through a transfer station having a rather lengthly transfer areaformed by parallel portions of the transfer web and a photoconductiveweb. To prevent what the reference termed "vacuum hole printout", theeffective position of the holes is gradually moved to differentlocations during passage through the transfer zone. Whatever theeffectiveness of this solution for the apparatus shown, it would not beuseful with the relatively small transfer zone formed by a transfer drumwith either an image carrying web or drum.

The Bothner apparatus shows a transfer drum having an aluminum base witha polyurethane coating of intermediate conductivity. The intermediateconductivity allows the creation of a relatively strong transferelectric field without electrical breakdown in the nip. It is believedthat the failure to transfer toner over a vacuum hole is due to lack ofcontinuity of the electric field in that region when a less conductive,for example, a dry transfer sheet is being used.

DISCLOSURE OF THE INVENTION

It is the object of the invention to provide an apparatus fortransferring electrostatically held toner images to a receiving sheet,which receiving sheet is held by a vacuum to a transfer member, with areduction of the aforementioned image defect associated with vacuumholes.

This and other objects are accomplished by a transfer member which has aline of closely spaced laser drilled holes.

According to a preferred embodiment each of the laser drilled holes isgenerally conical in shape, that is, it is larger at the inner surfaceof the outer layer of the transfer member than it is at the outersurface. With such a configuration the narrow opening at the top of thehole provides electrical continuity for the electric field in thevicinity of the hole but the wider lower portion of the hole keeps thehole from clogging up with paper dust, toner particles, fusing oil, andthe like.

It is also an object of the invention to provide a method for making atransfer drum or member that, in use, has a reduction in theaforementioned image defect associated with vacuum holes.

This and other objects are accomplished by the steps of forming a layerof appropriate conductivity material on the surface of a core, formingvacuum opening means in the core and laser drilling in said layer aplurality of small holes communicating with said opening means.

According to a preferred embodiment of the inventive method, the openingmeans is formed in the core. Then, flow preventing means is applied tothe core to prevent a flow of material into the opening means. The toplayer is coated on top of the core providing a layer partly defined bythe flow preventing means. The top layer is then laser drilled.

According to a another preferred embodiment, holes that are generallyconically shaped are formed in the transfer drum by three alternativemethods.

In the first method a short focal length lens is used to focus the laserbeam at a position in the vicinity of the outer surface of the outerlayer of the drum. The focal length of the lens is such that the beamspreads as it approaches the inner surface of the layer providing aconically shaped hole.

According to a second alternative method, the direction of a laser beamis rotated relative to the outer surface of the outer layer of the drumaround a position in the vicinity of the outer surface to form agenerally conically shaped hole having a neck at that position.

A third alternative method of forming generally conically shaped vacuumholes is to form the exterior layer of the drum on a generallyreflective core, for example, a normal aluminum core. As the hole isdrilled with the laser, laser light will reflect off the aluminum coredestroying some of the layer material next to the core thereby forming agenerally conically shaped hole. According to this method it isgenerally preferable to form the vacuum opening means in the core afterformation of the conically shaped hole.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the preferred embodiment of the inventionpresented below reference is made to the accompanying drawings, inwhich:

FIG. 1 is a schematic side view of a printer constructed according tothe invention, with many parts eliminated for clarity of illustration.

FIG. 2 is a top view of a portion of a transfer apparatus in which theinvention is usable.

FIG. 3 is a cross-section of a transfer drum shown in FIG. 2.

FIG. 4 is a graph illustrating the relationship of vacuum hole size, thepresence of artifacts and the surface resistance of the receiving sheet.

FIG. 5 is a schematic top view of a portion of a row of vacuum holesconstructed according to the invention showing the top of the vacuumholes in solid lines and the bottoms of the vacuum holes in phantom.

FIG. 6 is a partially schematic side cross-section illustratingmanufacture of a vacuum hole according to the invention.

FIGS. 7, 8 and 9 are side sections similar to FIG. 6 illustrating analternative method of forming a vacuum hole, which vacuum hole is shownin FIG. 9.

FIGS. 10, 11 and 12 are side sections similar to FIG. 6, illustratinganother embodiment of a method of making a vacuum hole, which vacuumhole is shown in FIG. 12.

FIG. 13 is a side cross-section similar to FIG. 6 illustrating anotherembodiment of making a vacuum hole according to the invention.

BEST MODE OF CARRYING OUT THE INVENTION

According to FIG. 1 a film core portion of a copier or printer includesan image bearing member, for example, an endless electrophotoconductiveweb 1 entrained about a series of primary rollers 2, 3, 4 and 5, andother supporting structure, for example, film skis 6.

Web 1 is driven through a series of electrophotographic stationsgenerally well-known in the art. More specifically, a uniform charge islaid down on the web 1 by a charging station 7. The uniformly chargedweb moves around printheld roller 2 which is directly opposite an LEDprinthead 8 which LED printhead exposes the web 1 in a manner well-knownin the art. The web then moves into operative relation with anelectrometer 9 which senses the level of a charge existing afterexposure of the web by printhead 8, to help control the process.

The web then moves into operative relation with a series of toning ordeveloping stations 10, 11, 12 and 13. Each image created by printhead 8is toned by one of the toning stations. After being toned the web passesa magnetic scavenger 14 which removes excess iron particles picked up inthe toning process. After the electrostatic image has been toned the webpasses under a densitometer 15 which measures the density of the tonerimage also for use in controlling the process. The toner image thenproceeds to a transfer station 16 where the image is transferred to atransfer surface of a receiving sheet carried by a transfer drum 18.

The transfer drum 18 includes vacuum holes 19 (FIGS. 2-3) for securingthe receiving sheet thereto for repeated presentations to web 1. Thetransfer drum 18 cooperates with web 1 to incrementally bring thereceiving sheet and the toner image into transfer relation so that thetoner image is transferred to the receiving sheet. As is well known inthe art, this is generally accomplished in the presence of an electricfield which is created by biasing the transfer drum by a suitablebiasing means, for example, electrical source 70, compared to theconductive layer of the web 1 or to a backing roller 20 for the web.This process has been well-known in the art for many years, see forexample, U.S. Pat. No. 3,702,482. Although either the web 1 or the drum18 could be at ground, conventionally the conductive backing is atground and the drum at a relatively high voltage. For example, if thetoner to be transferred is positively charged, the drum can be biased to-3000 V by electrical source 70.

As thoroughly discussed in U.S. Pat. No. 4,712,906, cited above, whenthe apparatus is operating in a multi-image mode, for example, amulticolor mode, consecutive images or pairs of images are toned withdifferent colored toners using the different toning stations 10-13.These consecutive images are transferred in registry to the receivingsheet as it repeatedly is brought into transfer relation with the web 1by the drum 18. After the transfer operation is complete, the receivingsheet is allowed to follow the web, for example, by removing the vacuumholding it to the drum 18 or by stripping the sheet with a skive, otherconventional stripping mechanism, or both. The receiving sheet isseparated from the web with the aid of an electrostatic sheet transportmechanism 21 and is transported to a fuser 40. The web is then cleanedby the application of a neutralizing corona and a neutralizing eraselamp and a magnetic brush cleaning mechanism all located at a cleaningstation 22.

The transfer drum 18 is driven by a motor 37, the drum 18 in turndriving the web 1 through a sprocket 32 which engages perforations 30(FIG. 2). The sprocket 32 also forms part of a registration and timingsystem which includes a sprocket 31 on printhead roller 2 which sprocketis linked to an encoder 33. The encoder 33 feeds signals indicative ofthe angular position of sprocket 31 to a drive 34 for the printhead 8which drive 34 times the application of information from an informationsource 35 to the printhead 8.

After the receiving sheet leaves the fuser 40 it can go directly to anoutput tray 41 or be deflected by a deflector 45 into a duplex pathaccording to the position of deflector 45, the position of which iscontrolled by the logic of the apparatus through means not shown. Theduplex path moves the sheet by rollers and guides directing it firstthrough a passive deflector 46 into turn-around rollers 50. Turn-aroundrollers 50 are independently driven to drive the receiving sheet intoturn-around guide means 51 until the trailing edge thereof has beensensed by an appropriate sensor, not shown, to have passed passivediverter 46. Once the trailing edge has passed passive diverter 46 theturn-around rollers 50 are reversed and the receiving sheet is driven byrollers 50 and other sets of drive rollers 52, 53, and 54 back to aposition upstream of the transfer station 16. The receiving sheet canpass through registration mechanisms for correcting for skew, crosstrackmisalignment and in-track misalignment and ultimately stop at alignmentrollers 55.

Transfer station 16 receives sheets from any of three sources. First, itcan receive sheets of one particular size from a first supply 25, whichfirst supply may include, for example, letter size sheets being fed withtheir short dimension parallel with the direction of feed. Second, itmay receive sheets from a second supply 26, which, for example, mayinclude ledger size sheets with their long dimension parallel to thedirection of movement. Third, the transfer station 16 may receive sheetsfrom the duplex path as controlled by rollers 55 which may includeeither size sheet and would already contain a fused image on its upperside. The receiving sheets from whatever source, stop against timingrollers 17. In response to a signal from the logic and control of theapparatus, not shown, timing rollers 17 accelerate to drive thereceiving sheet into the nip between the transfer drum 18 and the web 1as the first toner image to be transferred approaches the nip.

The duplex path is of a length that takes multiple sheets at one timedepending on the length of the sheets. For example, four letter sizesheets may be in the duplex path at one time or two ledger size sheets.If the printer is printing different images on different sheets, thelogic and control of the apparatus must supply the necessary programmingto the exposure and toning stations so that the sheets ultimately fed tothe output tray 41 are in the correct order considering the number ofsheets that must be in the duplex path. Such programming is known in theart, see, for example, U.S. Pat. No. 4,453,841 (Mead).

Transfer drum 18 is best seen in FIGS. 2 and 3. According to FIG. 2,vacuum holes 19 are positioned across the length of drum 18 to grip theleading edge of a receiving sheet. Vacuum is applied to the holes from asource of vacuum shown schematically as 80 through suitable conduits andvalves, some of which are not shown. U.S. Pat. No. 4,712,906 isincorporated by reference herein and shows more details of a suitablemechanism for applying and releasing the vacuum at the appropriate timesfor the holes gripping the leading edges of receiving sheets.

The drum 18 has an aluminum core and a polyurethane outer layer.Preferably, the polyurethane is of an intermediate conductivity, forexample, it may have a resistivity of 5×10⁹ ohm/cm. Transfer drumshaving an outer layer or layers of intermediate conductivity arewell-known and have certain advantages over drums having greaterconductivity. The outer layer is shown as a single layer, but can beformed of more than one layer. See, for example, U.S. Pat. No.3,781,105, Meagher, issued Dec. 25, 1973 for a discussion of some of theadvantages of intermediate conductivity transfer drums and illustratinguse of a two outer layer drum. The polyurethane layer is conductive inthe sense that it helps establish the electrical field urging transfer.

As seen in FIG. 3, vacuum holes 19 grip the leading edge of a firstletter sized receiving sheet 66 which encompasses slightly less thanhalf the circumference of the drum 18. The leading edge of a secondletter size sheet 67 is gripped by another row of vacuum holes 39. Formany grades of paper, vacuum holes for the leading edge are adequate.However, for best holding of a wide grade of materials, includingtransparency stock, vacuum holes 29 located along the trailing edge ofthe sheets assist in the holding process, preventing creep of thereceiving sheet on the drum surface and thereby preventingmisregistration of images. Additionally, a set of vacuum holes 59 (FIG.2) can be positioned along one or both lateral edges of the image areasto provide additional holding force.

If a ledger sized receiving sheet is to be used, the leading edge isstill attached using vacuum holes 19, but the sheet will stretch acrossone row of holes 29 and the row of holes 39 ending up short of thesecond row of holes 29. To secure the trailing edge of ledger sheets anadditional row of holes 49 is provided. If the trailing edge of othersizes of sheets (for example, legal size) is to be secured, additionalrows of holes for the additional trailing edges will be necessary.

Thus, even without the holes securing the trailing edges, at least onerow of vacuum holes will lie underneath the primary image area duringthe transfer process of a ledger size sheet. With the additional rows ofholes to secure the trailing edge of sheets, the number of holes ismultiplied. Under some conditions, the vacuum holes do not have anadverse effect on the final image. However, for many conditions,especially with a dry receiving sheet, for example, a sheet that hasbeen through a fuser once and is now receiving the second side of aduplex copy or a resin sheet, as is used to make a transparency,insufficient transfer is present in the portion of the sheet overlyingthe vacuum holes. This shows up on a white receiving sheet as a whitespot in the image. This phenomena is believed to be due to the fact thattransfer is accomplished primarily by a relatively strong electric fieldbetween the surface of the drum 18 and a conductive backing for theweb 1. In a humid environment, the paper is conductive and provides somecontinuity of the field over the holes. In dry conditions, the receivingsheet is less conductive and that field loses continuity over the holes.The toner does not transfer, staying on the surface of web 1.

Extremely small vacuum holes do not disrupt the transfer field enough tocause objectionable visual artifacts. The diameter of the vacuum holethat does not show a visible artifact varies inversely with theresistance of the receiving sheets. This is demonstrated in FIG. 4 wherethe diameter of a vacuum hole which is at the threshold of defectvisibility is plotted against the surface resistivity of the receivingsheet. A normal sheet of paper in a relatively humid environment may notshow a defect with a vacuum hole as large as 3.0 mm or larger. However,a resin based sheet commonly used for transparencies may still show adefect with holes at or below 0.4 mm in diameter. To handle a variety ofpaper receivers in the most common dry conditions, the hole should havea diameter less than 1.0 mm. However, for highest quality results invery dry conditions, especially with duplex copies, 0.5 mm to 0.65 mmdiameter holes are preferred.

To avoid any visual defects in the least conductive transparencies, holediameters less than 0.4 mm are necessary. The problem withtransparencies can be treated in several ways. Some transparency stockis more conductive, e.g., 10¹³ -10¹⁴ ohms/square in resistivity. Suchstock can be used with holes between 0.50 and 0.65 mm without theartifact. Even with less conductive stock, the defect with an opening0.5 to 0.65 mm in a transparency is a very small defect. If, in theapparatus shown in FIG. 1, most transparency reproductions are lettersize, the defect may only occur in the margin of transparencies, andbeing small may be acceptable. Alternatively, very small 0.4 mm openingscan be used. Some of the preferred embodiments of the invention produceopenings that small that will not clog in a relatively clean machineenvironment. For most applications, however, the former approach with0.5 to 0.65 mm openings and more conductive transparency stock ispreferred.

As the size of the vacuum hole is reduced, the holding power of thevacuum is also reduced, the holes have a tendency to plug up, andmanufactureability of the hole becomes a problem. According to theinvention, we have solved these problems by providing a large number ofsmall laser drilled holes, shown greatly magnified in FIG. 5.Preferably, they are placed in a straight line and as close together aspossible while maintaining the structural integrity of the outer layerof the transfer drum. For example, with relatively precise laserdrilling in excess of 4 holes 1 mm in diameter can be obtained to thecentimeter. With smaller sized holes, more density can be obtained.

Preferably, to prevent clogging of the holes with paper dust, tonerparticles, fusing oil, and the like, the holes have been drilled in agenerally conical shape. With this conical shape the narrow portion ofthe hole shown in solid circles in FIG. 5 provides the continuity forthe transfer field while the rest of the hole, being gradually larger,as it goes toward the inside of the drum is less restricting and lesslikely to clog with particles. Using this structure we have found thatsubstantial vacuum force can be consistently applied with holes having adiameter (at their neck) even less than a 0.5 to 0.65 mm range. Withinthis range, electrical continuity of the field is excellent for highresolution images.

FIG. 6 illustrates a method of manufacturing such small holes with aconical shape. An aluminum core 51 has a vacuum opening 52 ofsubstantial size, for example, 5 mm. It can be drilled by conventionaltechniques or formed in the original core manufacturing process. A layer53 of at least intermediate conductivity, for example, a layer ofpolyurethane having conductive additives making its resistance in theneighborhood of 5×10⁹ ohm-cm, is coated or otherwise applied to thesurface of aluminum core 51. Layer 53 may, for example, be 6 mm thick.

A laser 55, generally of the type commonly used for fine drilling, wouldordinarily have a fairly long focal length lens, say, 120 mm toconcentrate the energy of the laser through a substantial thickness ofmaterial. However, in this instance a relatively short focal length lens56, for example, a 60 mm lens, is used to focus a beam 57 at a positionat or just below the surface of layer 53. The spread of the laser beamfrom lens 56 is an inverse function of its focal length. Therefore, witha short focal length lens the beam 57 spreads substantially as it passesthrough material 53 thereby forming a generally conically shaped hole58.

Commercially available laser drilling devices commonly focus their beamsto as small as 0.1 mm. As explained above, for best results inmaintaining a vacuum, the vacuum hole 58 should have a smallest diametersomewhat larger than 0.1 mm. The commercial laser drilling apparatusautomatically moves the laser beam in a circle to form holes of diameterlarger than the size of the beam. That approach is also effective with ashort focal length lens 56 to create a generally conical hole. Formanufacturing tolerance reasons the point of focus may purposely beplaced slightly below the outer surface of layer 53. Thus, the conicalhole may in fact have its smallest diameter slightly below the surface.The term "conical" will be used herein to describe any shape which isgenerally larger at its base than it is at its top.

Prior vacuum holes in transfer drums have been manufactured by drillingholes, for example, 5 mm in diameter, directly through both layer 53 andlayer 51 thereby forming essentially a single hole through bothmaterials. However, direct mechanical drilling of holes smaller than 1mm through materials as different as polyurethane layer 53 and aluminumlayer 51 has proven to be quite difficult. Similarly, straight laserdrilling through both surfaces of both materials have a tendency tocreate substantial heat in drilling through the aluminum which heat hasan adverse effect on the adhesion between the polyurethane and thealuminum. Therefore, in practicing the method illustrated in FIG. 6,vacuum opening 52 is drilled or formed by conventional means prior tothe application of layer 53.

FIGS. 7, 8 and 9 show a method of providing the starting product for theprocess shown in FIG. 6. According to FIG. 7 the aluminum core has beendrilled with vacuum opening 52. A piece of thin tape 60 is placed overvacuum opening 52. According to FIG. 8 the polyurethane is then moldedor otherwise formed on top of the core 51 with the tape 60 preventingthe polyurethane from sinking into opening 52 creating depressions inthe surface and clogging opening 52. According to FIG. 9 then a conicalhole 58 is drilled through both layer 53 and tape 60. To preventdamaging heating of the interface between the layers, the tape is chosento not reflect the laser radiation. A conventional foam molding plugcould also be used.

FIGS. 10, 11 and 12 show another embodiment of the method for forming agenerally conical vacuum hole in layer 53. According to FIG. 10, aconventional laser beam 57 is applied to layer 53. Layer 53 has beencoated on aluminum core 51 but no vacuum opening has been drilled incore 51. Because of the somewhat diffuse reflectivity of the surface ofcore 51 some of laser beam 57 is reflected at substantial angles fromthat of the incoming beam. This reflection has a tendency to destroyportions of layer 53 adjacent the core 51 forming a generally conicalvacuum hole 58. Care must be taken to not overheat the layer interfaceor the bond between the polyurethane and the aluminum may be damagedaround the hole.

Laser beam 57 is shown as a parallel beam. However, all presentcommercial laser drilling beams have some spread even with use of arelatively long focal length lens. This small spread of itself willdirectly form a vacuum hole only marginally preferable to a strictlycylindrical hole. However, with the additional spreading from theportion of the beam reflected off the aluminum surface, a conical holewith a base in excess of 11/2 the diameter of the neck can be formed.

Aluminum core 51 must now be drilled quite carefully from the bottom toform an appropriate vacuum hole. This is shown in FIG. 11 using a largebit 80 which only slightly removes portions of layer 53 forming the endproduct shown in FIG. 12. This particular process has the disadvantageof requiring drilling of opening 52 from the rear of the core, which inmany instances is inconvenient. It also requires quite precise drillingto avoid damage to layer 53.

As a similar alternative to this latter process, opening 52 can bedrilled before applying layer 53 as in FIGS. 7-9. An appropriatelyreflective surface, for example, the surface of an aluminum plug isplaced in opening 52 for the laser drilling process itself. The aluminumplug reflects the laser beam as in FIG. 10 and is removed after thedrilling process.

FIG. 13 shows still another alternative approach to laser drilling aconical vacuum hole. According to FIG. 13, either the laser or the core51 is tilted so that low spread laser beam 57 is rotated about aposition 68 to form a generally conical vacuum hole in layer 53.

With all of these structures the end product is a hole that has a smallenough opening at its narrowest point to reduce the usual discontinuityin the transfer electrical field. At the same time because that narrowportion of the opening is not as thick as the layer 53, the opening isless likely to clog with paper dust and toner particles. If, as ispreferred, the vacuum holes are placed as close together as possible ina straight line, and if there is substantial spread in the hole as itapproaches the inner surface of layer 53, the bases of the holes willoverlap, see FIG. 5. Because of this the vacuum opening 52 mayappropriately be a long narrow opening that communicates with a numberof vacuum holes. Similarly, in the manufacturing process described withregard to FIGS. 7, 8 and 9, a single tape 60 would cover an opening oropenings that communicate with a number of vacuum holes.

Although any even slight spreading of the hole as it extends deeper inthe drum has a positive effect on reducing clogging, for best results,the diameter of the hole at the interface between layer 53 and vacuumopening 52 should be greater than 11/2 times that of the diameter at theneck or narrowest portion of the hole. With a 6 mm thick layer 53, thewider diameter can be greater than 3 times that of the narrower,creating what appears to be a long narrow groove in the rear of layer53.

We have found that when vacuum holes are not placed in a straight lineadjacent the edge of the paper to be held, the holes that are furtherfrom the edge have less holding effect. Therefore, for best results, thestraight line of holes shown in FIG. 5 should be placed as close astolerances will allow to the edge of the paper to be secured and theline should be parallel to that edge, i.e., parallel to the axis ofrotation of the drum.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention as described hereinabove and as defined in the appendedclaims.

We claim:
 1. Apparatus for transferring a toner image from animage-bearing member to a receiving sheet, said apparatus comprising:atransfer member having a surface and having vacuum opening means to saidsurface, means for applying a vacuum to said vacuum opening means tohold a receiving sheet on the surface of the transfer member, and meansfor creating an electric field urging a toner image toward a receivingsheet held on said surface, characterized in that said vacuum openingmeans include a line of closely spaced, laser drilled holes, said holeshaving a diameter at their narrowest point of 0.65 mm or less and beingspaced to provide a concentration of at least 4 to a centimeter. 2.Apparatus for transferring a toner image from an image-bearing member toa receiving sheet, said apparatus comprising:a transfer drum having acylindrical outer surface and having vacuum opening means to saidsurface, means for applying a vacuum to said vacuum opening means tohold a receiving sheet on the surface of the transfer drum, and meansassociated with said outer surface for creating an electric field urginga toner image toward said receiving sheet, characterized in that saidvacuum opening means include a plurality of laser drilled holes, eachhole being narrow at said surface and wider deeper in the drum. 3.Apparatus for transferring a toner image from an image bearing member toa receiving sheet, said apparatus comprising:a transfer drum having anaxis and having a cylindrical conductive outer surface defined by anouter layer of intermediate conductivity, disposed on a conductive metalcore, and having vacuum opening means through said core and outer layerto said surface, means for applying a vacuum to said vacuum openingmeans to hold a receiving sheet on the surface of the transfer drum, andmeans for creating an electric field urging a toner image toward saidreceiving sheet, characterized in that said vacuum opening means includeopening means in said core and a line of closely spaced, laser drilledholes through said outer layer, said holes communicating with saidopening means in said core and being generally conical in shape beingnarrower adjacent the surface and wider at the interface between theouter layer and the opening means in the core.
 4. Apparatus according toclaim 3 wherein the diameter of each hole at its narrowest point is lessthan 1 mm.
 5. Apparatus according to claim 3 wherein the diameter ofeach hole at its narrowest point is 0.65 mm or less and the diameter ofeach hole at said interface is greater than one and one half times saiddiameter at the narrowest point.
 6. Apparatus according to claim 3wherein said line of holes is substantially parallel to the axis of thedrum and the holes are sufficiently closely spaced that they overlapeach other at said interface.
 7. Apparatus according to claim 3 whereinthere are at least four holes to a centimeter of said line.
 8. Apparatusaccording to claim 6 wherein there are at least four holes to acentimeter of said line, and the diameter of each hole at its narrowestpoint is 0.65 mm or less.
 9. Apparatus for forming multicolor tonerimages on a receiving sheet, said apparatus includingmeans for forming aseries of electrostatic images on an image bearing member, means fortoning said electrostatic images with toners of different colors,apparatus for transferring a plurality of said images to a receivingsheet in registry with each other image, to form a multicolor image onsaid receiving sheet, wherein said transferring apparatus is constructedaccording to claim
 1. 10. Apparatus according to claim 9 wherein saidtransfer member is a transfer drum having at least two sets of vacuumholes formed in straight lines running parallel to its axis, one to holdthe leading edge of each of two receiving sheets.